In general, LNG refers to a colorless, transparent, and ultra-low temperature liquid obtained by cooling methane-based natural gas at about −163° C. and reducing the volume thereof to 1/600.
LNG carriers to carry LNG or floating offshore platforms to produce and store LNG are provided with a cargo tank capable of containing and storing the LNG at an ultra-low temperature. The LNG cargo tank uses a membrane sheet as a primary barrier and uses a triplex as a secondary barrier. Herein, the triplex for the secondary barrier includes a rigid triplex and a supple triplex. The rigid triplex is previously attached on the surface of an insulation panel during a manufacturing process, and the supple triplex is attached over the rigid triplex through an adhesive such as epoxy glue.
FIG. 1A illustrates a conventional LNG cargo tank 1. Referring to FIG. 1A, the cargo tank 1 of a ship or floating offshore platform includes insulation panels 2 installed on the bottom, ceiling, or walls of the inner surface thereof, wherein the insulation panel 2 is a kind of radiation panel which is previously manufactured.
The insulation panels 2 may include a lower insulation panel 2a, a rigid triplex 5, and an upper insulation panel 2b. The lower insulation panel 2a is attached and fixed to the walls and the like of the cargo tank 1 through epoxy mastic and stud bolts. The rigid triplex is formed over the lower insulation panel 2a. The upper insulation panel 2b is disposed over the rigid triplex 5 and having a smaller horizontal area than the lower insulation panel 2a. 
Further, a flat joint 2c formed of glass wool and having pores is inserted between the lower insulation panels 2a adjacent to each other.
Further, between the upper insulation panels 2b is formed a working area 3.
In the working area 3, a supple or flexible triplex 7 is attached to the top surface of the rigid triplex 5 with an adhesive layer 6 interposed therebetween, and a top bridge pad (not illustrated) is installed.
Referring to FIG. 1B, the rigid triplex 5 and the supple triplex 7 of the conventional LNG cargo tank are formed of composite materials in which a first layer A, a second layer C, and an intermediate B are arranged in a sandwich structure or stacked structure. The first and second layers A and C are formed of glass cloth, and the intermediate layer B is formed of aluminum. In this stacked structure, the intermediate layer B serves for liquid tightness and/or air tightness, and the first and second layers A and C serve to reinforce the intermediate layer B. The first and second layers A and C are attached to each other with the intermediate layer B interposed therebetween, and the outer surface of the first or second layer A or C is exposed to the external environment.
The rigid triplex 5 of the conventional LNG cargo tank is strictly managed after the rigid triplex 5 is manufactured to be coupled to the lower insulation panel 2A. Furthermore, the rigid triplex 5 is managed without a flexural deformation even while it is shipped for the use. Therefore, gas and/or liquid L rarely leaks from the rigid triplex 5.
On the other hand, although the supple triplex 7 of the conventional LNG cargo is strictly managed, it is highly likely that gas and/or liquid L leaks from the supply triplex 7, because the supple triplex 7 is rolled in a manufacturing process and unrolled to use in the field.
For example, when the secondary barrier is constructed in the cargo tank using the supple triplex 7, the second layer C of the rigid triplex 5 and the first layer A of the supple triplex 7 are bonded to each other. Under construction, a portion where an interlayer adhesive is not sufficiently impregnated may become weak due to a manufacturing error or careless management of the supple triplex 7 or the method in which the supple triplex 7 is rolled and then unrolled to use. In this case, a leakage path may be formed between the second layer C and the intermediate layer B or the first layer A and the intermediate layer B. Along the leakage path formed in such a manner, gas and/or leakage L may leak. The leakage may cause a problem between the first layer A and the intermediate layer B of the supple triplex 7.
Furthermore, although a crack is formed between the second layer C and the intermediate layer B of the supple triplex 7 due to repeated thermal loads, liquid tightness and/or air tightness is maintained by the intermediate layer B. Therefore, the gas and/or liquid L inside the cargo tank does not leak toward the pores or the outside of the cargo tank.
On the other hand, the crack may form a leakage path P1 between the first layer A and the intermediate layer B of the supple triplex 7, and may form another leakage path P2 extended from the leakage path P1 in the thickness direction of the first layer A. That is, when the viscosity of the interlayer adhesive impregnated in the composite materials forming the supple triplex 7 is high and the supple triplex 7 is subjected to repeated thermal loads, a crack may occur due to a difference in thermal expansion coefficient between glass cloth and resin, thereby forming the above-described leakage paths P1 and P2.
Therefore, when an operation of attaching the supple triplex 7 in a strip or belt shape is performed in the working area 3 described with reference to FIG. 1 so as to construct the secondary barrier of the LNG cargo hole, gas and/or liquid L inside the cargo tank may leak to the outside of the cargo tank through the leakage paths P1 and P2 and the pores of the flat joint 2C. Accordingly, the liquid tightness and/or air tightness required by the secondary barrier may not be achieved.
For example, Korean Patent Laid-Open Publication No. 10-2009-0132534 published on Dec. 30, 2009 and commonly owned application discloses an insulation structure of an LNG carrier cargo tank which improves reliability for repeated thermal loads and has an improved sealing force.